Linear compressor

ABSTRACT

A linear compressor is provided. The linear compressor may include a cylinder that forms a compression space for a refrigerant, a piston that reciprocates in an axial direction inside of the cylinder, and a linear motor that supplies power to the piston. The linear motor may include an outer stator including a first stator magnetic pole, a second stator magnetic pole, and an opening defined between the first stator magnetic pole and the second stator magnetic pole; an inner stator disposed apart from the outer stator to form an air gap therebetween; and a permanent magnet movably disposed in the air gap between the outer stator and the inner stator and having three poles. The three poles may include two end magnetic poles, and a central magnetic pole disposed between the two end magnetic poles. The piston may be moveable by a stroke between a top dead center position and a bottom dead center position, and a length of the first stator magnetic pole or the second stator magnetic pole may be greater than a length of the stroke.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority under 35 U.S.C. 119 and 35U.S.C. 365 to Korean Patent Application No. 10-2013-0075512, filed inKorea on Jun. 28, 2013, which is hereby incorporated by reference in itsentirety.

BACKGROUND

1. Field

A linear compressor is disclosed herein.

2. Background

In general, compressors may be mechanisms that receive power from powergeneration devices, such as electric motors or turbines, to compressair, refrigerants, or other working gases, thereby increasing a pressureof the working gas. Compressors are widely used in home appliances orindustrial machineries, such as refrigerators and air-conditioners.

Compressors may be largely classified into reciprocating compressors, inwhich a compression space, into and from which a working gas, such as arefrigerant, is suctioned and discharged, is defined between a pistonand a cylinder to compress the refrigerant while the piston is linearlyreciprocated within the cylinder; rotary compressors, in which acompression space, into and from which a working gas, such as arefrigerant, is suctioned and discharged, is defined between a roller,which is eccentrically rotated, and a cylinder to compress therefrigerant while the roller is eccentrically rotated along an innerwall of the cylinder; and scroll compressors in which a compressionspace, into and from which a working gas, such as a refrigerant, issuctioned and discharged, is defined between an orbiting scroll and afixed scroll to compress the refrigerant while the orbiting scroll isrotated along the fixed scroll. In recent years, among the reciprocatingcompressors, linear compressors having a simple structure in which apiston is directly connected to a drive motor, which is linearlyreciprocated, to improve compression efficiency without mechanical lossdue to switching in moving, are being actively developed. Generally,such a linear compressor is configured to suction and compress arefrigerant while a piston is linearly reciprocated within a cylinder bya linear motor in a sealed shell, thereby discharging the compressedrefrigerant.

The linear motor has a structure in which a permanent magnet is disposedbetween an inner stator and an outer stator. The permanent magnet may belinearly reciprocated by a mutual electromagnetic force between thepermanent magnet and the inner (or outer) stator. Also, as the permanentmagnet is operated in a state in which the permanent magnet is connectedto the piston, the refrigerant may be suctioned and compressed while thepiston is linearly reciprocated within the cylinder and then bedischarged.

A linear compressor according to the related art is disclosed in KoreanPatent Publication No. 10-2010-0010421. The linear compressor accordingto the related art includes a linear motor, which is provided with anouter stator having a core and a coil-wound body, an inner stator, and apermanent magnet. One end of a piston is connected to the permanentmagnet. The permanent magnet may include one magnet having a singlepolarity, and may be a rare-earth magnet.

When the permanent magnet is linearly reciprocated by mutualelectromagnetic force between the inner stator and the outer stator, thepiston linearly reciprocates in a cylinder along with the permanentmagnet. However, rare earth metals are expensive.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described in detail with reference to the followingdrawings in which like reference numerals refer to like elements, andwherein:

FIG. 1 is a cross-sectional view of a linear compressor according to anembodiment;

FIG. 2 is an enlarged view of a portion “A” of the linear compressor ofFIG. 1;

FIGS. 3 and 4 are cross-sectional views illustrating a reciprocatingmotion of a permanent magnet in an axial direction according tooperation a linear motor of the linear compressor of FIG. 1;

FIGS. 5 and 6 are cross-sectional views schematically illustrating thelinear motor of FIGS. 3-4;

FIG. 7A illustrates magnetic flux in a linear motor having a magneticpole tip distance of T1, FIG. 7B illustrates magnetic flux in a linearmotor having a magnetic pole tip distance of T2, and FIG. 7C illustratesa magnitude of leakage magnetic flux in the linear motors of FIGS. 7Aand 7B;

FIG. 8 is a cross-sectional view of a linear motor illustrating aposition of a permanent magnet when a piston is positioned at a bottomdead center (BDC) position, according to an embodiment;

FIG. 9 is a cross-sectional view of a linear motor illustrating aposition of a permanent magnet when a piston is positioned at a top deadcenter (TDC) position, according to an embodiment;

FIG. 10 is a graph showing a magnitude of a thrust generated accordingto lengths of magnetic poles at both ends, in a permanent magnetaccording to an embodiment;

FIG. 11 is a graph showing variations in a cogging force according tolengths of magnetic poles at both ends, in a permanent magnet accordingto an embodiment;

FIG. 12 is a graph showing a magnitude of a thrust generated accordingto a length of a central magnetic pole, in a permanent magnet accordingto an embodiment; and

FIG. 13 is a graph showing variations in a cogging force according to alength of a central magnetic pole, in a permanent magnet according to anembodiment.

DETAILED DESCRIPTION

Hereinafter, embodiments will be described with reference toaccompanying drawings. However, the scope is not limited to embodimentsdisclosed herein, and thus, a person skilled in the art, who understoodthe scope, would easily suggest other embodiments within the same scopethereof.

FIG. 1 is a cross-sectional view of a linear compressor according to anembodiment. Referring to FIG. 1, the linear compressor 10 may include acylinder 120 disposed in a shell 100, a piston 130 that linearlyreciprocates inside the cylinder 120, and a motor assembly 200, whichmay be in the form of a linear motor, that exerts a drive force on thepiston 130. The shell 100 may include an upper shell and a lower shell.

The shell 100 may further include an inlet 110, through which arefrigerant may flow into the shell 100, and an outlet 105, throughwhich the refrigerant compressed inside the cylinder 120 may bedischarged from the shell 100. The refrigerant suctioned in through theinlet 101 may flow into the piston 130 via a suction muffler 140. Whilethe refrigerant is passing through the suction muffler 140, noise may bereduced.

The piston 130 may be made of a nonmagnetic material, such as analuminum-based material, for example, aluminum or aluminum alloy. As thepiston 130 may be made of the aluminum-based material, magnetic fluxgenerated in the motor assembly 200 may be delivered to the piston 130,thereby preventing the magnetic flux from being leaked outside of thepiston 130. The piston 130 may be formed by forging, for example.

The cylinder 120 may be made of a nonmagnetic material, such as analuminum-based material, for example, aluminum or aluminum alloy. Thecylinder 120 and the piston 130 may have a same material compositionratio, that is, type and composition ratio.

As the cylinder 120 may be made of the aluminum-based material, magneticflux generated in the motor assembly 200 may be delivered to thecylinder 120, thereby preventing the magnetic flux from being leakedoutside of the cylinder 120. The cylinder 120 may be formed by extrudedrod processing, for example.

The piston 130 and the cylinder 120 may be made of the same material,for example, aluminum, and thus, may have a same thermal expansioncoefficient. During operation of the linear compressor 10, ahigh-temperature environment (about 100

) may be created in the shell 100. At this time, the piston 130 and thecylinder 120 may have the same thermal expansion coefficient, and thus,may have a same amount of thermal deformation. As the piston 130 and thecylinder 120 may be thermally deformed in different amounts ordirections, it is possible to prevent interference with the cylinder 120during movement of the piston 130.

A compression space P to compress the refrigerant by the piston 130 maybe defined in the cylinder 120. A suction hole 131 a, through which therefrigerant may be introduced into the compression space P, may bedefined in the piston 130, and a suction valve 132 to selectively openthe suction hole 131 a may be disposed at a side of the suction hole 131a.

A discharge valve assembly 170, 172, and 174 to discharge therefrigerant compressed in the compression space P may be disposed at aside of the compression space P. That is, the compression space P may beformed between an end of the piston 130 and the discharge valve assembly170, 172, and 174.

The discharge valve assembly 170, 172, and 174 may include a dischargecover 172, in which a discharge space of the refrigerant may be defined;a discharge valve 170, which may be opened and introduce the refrigerantinto the discharge space when the pressure of the compression space P isnot less than a discharge pressure; and a valve spring 174, which may bedisposed between the discharge valve 170 and the discharge cover 172 toexert an elastic force in an axial direction. The term “axial direction”used herein may refer to a direction in which the piston linearlyreciprocates, that is, a substantially horizontal direction in FIG. 1,while the term “radial direction” may refer to a direction substantiallyperpendicular to the reciprocating direction of the piston 130, that is,a substantially vertical direction in FIG. 1.

The suction valve 132 may be disposed at a first side of the compressionspace P, and the discharge valve 170 may be disposed at a second side ofthe compression space P, that is, at an opposite side to the suctionvalve 132. While the piston 130 linearly reciprocates inside thecylinder 120, the suction valve 132 may be opened to allow therefrigerant to be introduced into the compression space P when thepressure of the compression space P is lower than the discharge pressureand not greater than a suction pressure. In contrast, when the pressureof the compression space P is not less than the suction pressure, therefrigerant of the compression space P may be compressed in a state inwhich the suction valve 132 is closed.

If the pressure of the compression space P is the discharge pressure orgreater, the valve spring 174 may be deformed to open the dischargevalve 170, and the refrigerant may be discharged from the compressionspace P into a discharge space of the discharge cover 172. Therefrigerant of the discharge space may flow into a loop pipe 178 via adischarge muffler 176. The discharge muffler 176 may reduce flow noiseof the compressed refrigerant, and the loop pipe 178 may guide thecompressed refrigerant to the outlet 105. The loop pipe 178 may becoupled to the discharge muffler 176 and curvedly extend to be coupledto the outlet 105.

The linear compressor 10 may further include a frame 110. The frame 110,which may fix the cylinder 120 within the shell 100, may be integrallyformed with the cylinder 120 or may be coupled to the cylinder 120 bymeans of a separate fastening member, for example. The discharge cover172 and the discharge muffler 176 may be coupled to the frame 110.

The motor assembly 200 may include an outer stator 210, which may befixed to the frame 110 and disposed so as to surround the cylinder 120,an inner stator 220 disposed apart from an inside of the outer stator210, and a permanent magnet 230 disposed in a space between the outerstator 210 and the inner stator 220. The permanent magnet 230 maylinearly reciprocate due to a mutual electromagnetic force between theouter stator 210 and the inner stator 220. The permanent magnet 230 mayinclude a single magnet having one pole facing the outer stator 210, ormultiple magnets having three poles facing the outer stator 210. In thecase of the permanent magnet 230 having three poles, one surface theremay have a polar distribution of N-S-N, and the other surface thereofmay have a polar distribution of S-N-S

The permanent magnet 230 may be coupled to the piston 130 by aconnection member 138. The connection member 138 may extend to thepermanent magnet 230 from an end of the piston 130. As the permanentmagnet 230 linearly moves, the piston 130 may linearly reciprocate in anaxial direction along with the permanent magnet 230.

The outer stator 210 may include a bobbin 213, a coil 215, and a statorcore 211. The coil 215 may be wound in a circumferential direction ofthe bobbin 213. The coil 215 may have a polygonal section, for example,a hexagonal section. The stator core 211 may be formed by stacking aplurality of laminations in a circumferential direction, and may bedisposed to surround the bobbin 213 and the coil 215.

A stator cover 240 may be disposed at a side of the outer stator 210. Afirst end of the outer stator 210 may be supported by the frame 110, anda second end of the outer stator 210 may be supported by the statorcover 240.

The inner stator 220 may be fixed to an outer circumference of thecylinder 120. The inner stator 220 may be formed by stacking a pluralityof laminations at an outer side of the cylinder 120 in a circumferentialdirection.

The linear compressor 10 may further include a supporter 135 thatsupports the piston 130, and a back cover 115 that extends toward theinlet 101 from the piston 130. The back cover 115 may be disposed tocover at least a portion of the suction muffler 140.

The linear compressor 10 may include a plurality of springs 151 and 155,a natural frequency of each of which may be adjusted so as to allow thepiston 130 to perform resonant motion. The plurality of springs 151 and155 may include a plurality of first springs 151 supported between thesupporter 135 and the stator cover 240, and a plurality of secondsprings 155 supported between the supporter 135 and the back cover 115.

The plurality of first springs 151 may be provided at both sides of thecylinder 120 or the piston 130, and the plurality of second springs 155may be provided at a front of the cylinder 120 or the piston 130. Theterm “front” used herein may refer to a direction oriented toward theinlet 101 from the piston 130. The term “rear” may refer to a directionoriented toward the discharge valve assembly 170, 172, and 174 from theinlet 101. These terms may also be equally used in the followingdescription.

A predetermined amount of oil may be stored on an inner bottom surfaceof the shell 100. An oil supply device 160 to pump oil may be providedin a lower portion of the shell 100. The oil supply device 160 may beoperated by vibration generated according to the linear reciprocatingmotion of the piston 130 to thereby pump the oil upward.

The linear compressor 10 may further include an oil supply pipe 165 thatguides flow of the oil from the oil supply device 160. The oil supplypipe 165 may extend from the oil supply device 160 to a space betweenthe cylinder 120 and the piston 130. The oil pumped from the oil supplydevice 160 may be supplied to the space between the cylinder 120 and thepiston 130 via the oil supply pipe 165, and perform cooling andlubricating operations.

FIG. 2 is an enlarged view of a portion “A” of the linear compressor ofFIG. 1. FIGS. 3 and 4 are cross-sectional views illustrating areciprocating motion of the permanent magnet in an axial directionaccording to operation of a linear motor of the linear compressor ofFIG. 1.

Referring to FIGS. 2 to 4, the outer stator 210 according to embodimentsmay include the stator core 211, in which the plurality of laminationsmay be stacked in the circumferential direction. The stator core 211 maybe configured such that a first core 211 a and a second core 211 b arecoupled at a coupling portion 211 c.

An accommodation space, in which the bobbin 213 and the coil 215 may bedisposed, may be defined in the stator core 211, and an opening 219 maybe provided at a side of the accommodation space. That is, the firstcore 211 a and the second core 211 b may be coupled, such that thestator core 211 has the opening 219 at a central portion thereof tothereby have a C-shape.

The first core 211 a may include a first stator magnetic pole 217 thatacts with the permanent magnet 230. The second core 211 b may include asecond stator magnetic pole 218 that acts with the permanent magnet 230.The first stator magnetic pole 217 and the second stator magnetic pole218 may be portions of the first and second cores 211 a and 211 b,respectively. The opening 219 may be a space between the first statormagnetic pole 217 and the second magnetic pole 218.

The permanent magnet 230 may be formed of a ferrite material, which maybe relatively inexpensive. The permanent magnet 230 may include multiplepoles 231, 232 and 233, polarities of which may be alternately arranged.The multiple poles 231, 232, and 233 may include a first pole 231, asecond pole 232, and a third pole 233, which may be coupled to eachother.

When a current is applied to the motor assembly 200, a current may flowthrough the coil 215, a magnetic flux may be formed around the coil 215by the current flowing through the coil 215, and the magnetic flux mayflow along the outer stator 210 and the inner stator 220 while forming aclosed circuit. The first stator magnetic pole 217 may form one of anN-pole or a S-pole, and the second stator magnetic pole 218 may form theother one of the N-pole or the S-pole (see solid line arrow A in FIG.5).

The multiple poles 231, 232, and 233 (permanent magnet 230) may linearlyreciprocate in an axial direction between the outer stator 210 and theinner stator 220 by means of an interaction force of the magnetic fluxflowing through the outer stator 210 and the inner stator 220 and themagnetic flux formed by the multiple poles 231, 232, and 233 (permanentmagnet 230). The piston 130 may move inside the cylinder 120 by motionsof the multiple poles 231, 232, and 233 (permanent magnet 230).

When the current flowing through the coil 215 changes its direction, adirection of the magnetic flux passing through the outer stator 210 andthe inner stator 220 may be changed. That is, in the above-describedexample, polarities of the first and second stators 217 and 218 may beinterchanged. Therefore, a movement direction of the multiple poles 231,232, and 233 (permanent magnet 230) may be reversed, and therefore, amovement direction of the piston 130 may also be changed. In this way,as the direction of the magnetic flux is changed repetitively, thepiston 130 may linearly reciprocate.

FIG. 3 illustrates a mode in which first spring 151 is elongated whenthe multiple poles 231, 232, and 233 (permanent magnet 230) move in afirst direction. FIG. 4 illustrates a mode in which the second spring151 is compressed when the multiple poles 231, 232, and 233 (permanentmagnet 230) move in a second direction.

The multiple poles 231, 232, and 233 (permanent magnet 230) and thepiston 130 may linearly reciprocate by repeating the modes of FIGS. 3and 4. For example, when the permanent magnet 230 is at a position shownin FIG. 3, the piston 130 is positioned at a bottom dead center (BDC)position, and when the permanent magnet 230 is at a position shown inFIG. 4, the piston 130 is positioned at a top dead center (TDC)position.

The term BDC may refer to a position when the piston 130 is at a lowestposition inside the cylinder 120, that is, a position when the piston130 is disposed farthest away from the compression space P. The term TDCmay refer to a position when the piston 130 is at a highest positioninside the cylinder 120, that is, a position when the piston 130 isdisposed closest to the compression space P.

Hereinafter, a structure of the motor assembly 200 will be more fullydescribed with reference to the drawings.

FIGS. 5 and 6 are cross-sectional views schematically illustrating thelinear motor of FIGS. 3-4. Referring to FIG. 5, according to anembodiment, the first stator magnetic pole 217 of the first core 211 aand the second stator magnetic pole 218 of the second core 211 b may bedisposed apart from each other with respect to the opening 219.

In more detail, a first tip 217 a may be provided at an end of the firststator magnetic pole 217, and a second tip 218 a may be provided at oron the second stator magnetic pole 218. The opening 219 may be formed bya separation between the first tip 217 a and the second tip 218 a. Anaxial direction length of the opening 219 may be defined as “T”, whichmay be a distance between the first tip 217 a and the second tip 218 a.

A gap between the outer stator 210 and the inner stator 220 may be anair gap. More specifically, the air gap may be a portion at which themagnetic flux generated in the outer stator 210 and the magnetic fluxgenerated in the permanent magnet 230 meet, and thus, a thrust for thepermanent magnet 230 may be formed by interaction of the magneticfluxes. A height of the air gap may be defined as “G”. As the permanentmagnet 230 reciprocates in the air gap, a thickness MT of the permanentmagnet 230 may be formed smaller than the height G of the air gap.

As described in FIG. 5, when a current is applied to the coil 215 so asto form the magnetic flux in a clockwise direction, a portion of themagnetic flux may pass through the first stator magnetic pole 217, thesecond stator magnetic pole 218, the permanent magnet 230, and the innerstator 220. A first portion of the magnetic flux may be referred to asan “air gap magnetic flux”. The air gap magnetic flux may generate athrust for the permanent magnet 230.

A second portion of the magnetic flux may be formed to pass through thefirst stator magnetic pole 217 from the second stator magnetic pole 218.The other portion of the magnetic flux is not helpful for generating athrust to act on the permanent magnet 230, and thus, may be referred toas “leakage magnetic flux (dotted arrow).

A relationship between the height G of the air gap and the axialdirection length T of the opening 219 is provided hereinbelow.

As described above, the magnetic flux may include the air gap magneticflux and the leakage magnetic flux. When one of the air gap magneticflux or the leakage magnetic flux increases, the other magnetic flux maydecrease, relatively.

A ratio between the air gap magnetic flux and the leakage magnetic fluxmay vary with a ratio between the height G of the air gap and the axialdirection length T of the opening 219. In more detail, as the gapbetween the outer stator 210 and the inner stator 220 increases as theheight G of the air gap increases, a magnitude of the magnetic fluxflowing into the inner stator 220 from the outer stator 210 decreases.That is, the magnitude of the air gap magnetic flux decreases.

As the gap between the outer stator 210 and the inner stator 220decreases as the axial direction length T of the opening 219 decreases,a magnitude of the magnetic flux flowing from one of the inner stator220 or the outer stator 210 into the other stator increases. That is,the magnitude of the air gap magnetic flux increases.

Therefore, to reduce the leakage magnetic flux and increase the air gapmagnetic flux relatively, the axial direction length T of the opening219 may be equal to or greater than the height G of the air gap. Thatis, T≧G may be established. Related effects may be confirmed in FIGS. 7Ato 7C.

FIG. 7A illustrates magnetic flux in a linear motor having a magneticpole tip distance of T1. FIG. 7B illustrates magnetic flux in a linearmotor having a magnetic pole tip distance of T2. FIG. 7C illustrates amagnitude of a leakage magnetic flux in the linear motor of FIGS. 7A and7B.

FIG. 7A illustrates a flow of the magnetic flux generated in the motorassembly 200 when the axial direction length of the opening 219 is T1,and FIG. 7B illustrates a flow of the magnetic flux generated in themotor assembly 200 when the axial direction length of the opening 219 isT2. T2 is greater than T1. For example, T1 may be approximately 3 mm andT2 may be approximately 9 mm. The air gaps in FIGS. 7A and 7B may have asame height G.

In FIGS. 7A and 7B, when a point where a first line in a radialdirection, which penetrates through a center of the opening 219, meetsthe inner stator 220 is defined as a zero point (O), a pointintersecting with a second line that connects the first and secondstator magnetic poles 217 and 218 may be defined as a first point (P1).A distance between the zero point O and the first point P1 maycorrespond to the height of the air gap. Also, when a point on thebobbin 213 at which the first line intersects with the coupling part 211c is defined in as a second point (P2), FIG. 7C illustrates a magneticflux that leaks from the motor assembly 200.

In more detail, as illustrated in FIG. 7A, in the height G of the airgap, if the opening 219 has a relatively small axial length, a leakagemagnetic flux of the magnetic flux generated in the outer stator 210,for example, a leakage magnetic flux of a positive (+) pole maysignificantly increase from the zero point O to the first point P1 toform a maximum leakage magnetic flux at the point P1. The leakagemagnetic flux may gradually decrease from the first point P1 to thesecond point P2.

Also, the leakage magnetic flux of the positive (+) pole may be switchedin direction to a negative (−) pole to significantly increase. Away fromthe second point P2, the magnitude of the leakage magnetic flux may havean approximately constant value (a constant magnetic flux). Herein, theterms “positive (+) pole” and “negative (−) pole” may denote leakagemagnetic flux directions opposite to each other. Also, the constantmagnetic flux may be a maximum magnetic flux of the negative (−) pole.

On the other hand, as illustrated in FIG. 7B, in the height G of the airgap, if the opening 219 has a relatively large axial length, a leakagemagnetic flux of the magnetic flux generated in the outer stator 210,for example, a leakage magnetic flux of a positive (+) pole may smoothlyincrease from the zero point O to the first point P1 to form a maximumleakage magnetic flux at the first point P1. The maximum magnetic fluxin FIG. 7B may have a value relatively less than that in FIG. 7A. Theleakage magnetic flux may gradually decrease from the first point P1 tothe second point P2.

The leakage magnetic flux of the positive (+) pole may be switched indirection to the negative (−) pole to significantly increase. Away fromthe first point P2, the magnitude of the leakage magnetic flux may havean approximately constant value (a constant magnetic flux). However, theconstant magnetic flux in FIG. 7B may have a value relatively greaterthan that in FIG. 7A.

As illustrated in FIG. 7C, with respect to the height G of thepredetermined air gap, the more the opening increases in length T, themore the maximum leakage magnetic flux, that is, the maximum magneticfluxes of the positive (+) and negative (−) poles decrease. Thus, alarger amount of thrust may be provided to the permanent magnet 230 toimprove the operation efficiency of the motor assembly 200.

FIG. 8 is a cross-sectional view of a linear motor illustrating aposition of a permanent magnet when a piston is positioned at the BDCposition, according to an embodiment. FIG. 9 is a cross-sectional viewof a linear motor illustrating a position of a permanent magnet when apiston is positioned at the TDC position, according to an embodiment.

Referring to FIGS. 5, 6, 8, and 9, the permanent magnet according toembodiments may include the plurality of poles 231, 232, and 233, whichmay be alternately arranged in polarity. The plurality of poles 231,232, and 233 may include the first pole 231, the second pole 232 coupledto the first pole 231, and the third pole 233 coupled to the second pole232.

The second pole 232 may be referred to as a “central magnetic pole”, andthe first and third poles 231 and 233 may be referred to as “both endmagnetic poles” in that the second pole 232 is disposed between thefirst and third poles 231 and 233.

The central magnetic pole 232 may have a length greater than a length ofeach of the both end magnetic poles 231. 233. A length of the centralmagnetic pole 232 may be defined as a length “MC”, a length of the firstpole 231 may be defined as a length “MF”, and a length of the third pole233 may be defined as a length “MR”. The lengths MF and MR may have thesame value. On the other hand, the lengths MF and MR may have valuesdifferent from each other so as to increase the thrust according to adesign of the compressor.

A first interface surface 235 may be disposed between the first pole 231and the second pole 232, and a second interface surface 236 may bedisposed between the second pole 232 and the third pole 233. The firstinterface surface 235 may be reciprocated within a range which is notout of a range of the first stator magnetic pole 217 with respect to acenter of the first stator magnetic pole 217, and the second interfacesurface 236 may be reciprocated within a range which is not out of arange of the second stator magnetic pole 218 with respect to a center ofthe second stator magnetic pole 218.

That is, the first interface surface 235 may be reciprocated in an axialdirection between both ends of the first stator magnetic pole 217 withrespect to the center of the first stator magnetic pole 217. Also, thesecond interface surface 236 may be reciprocated in the axial directionbetween both ends of the second stator magnetic pole 218 with respect tothe center of the second stator magnetic pole 218.

A force (thrust) pulled and pushed between polarities (an N pole or an Spole) of the first stator magnetic pole 217 and polarities of the firstand second poles 231 and 232 may occur. Also, as the force pulled andpushed between polarities (an N pole or an S pole) of the second statormagnetic pole 217 and polarities of the second and third poles 231 and232 may occur, the permanent magnet may be reciprocated.

The first and second poles 231 and 233 may have the same polarity. Thesecond pole 232 disposed between the first and third poles 231 and 233may have a polarity opposite to a polarity of each of the first andsecond poles 231 and 233. For example, if each of the first and thirdpoles 231 and 233 is a N pole, the second pole 232 may be a S pole. Ifeach of the first and third poles 231 and 233 is a S pole, the secondpole 232 may be a N pole.

A structure, in which two poles acting on each other with respect to thefirst stator magnetic pole 217 are disposed, and the other two polesacting on each other with respect to the second stator magnetic pole 218are disposed, may be provided to generate a larger amount of thrust onthe permanent magnet 230. The two poles acting on each other may havethe same length, and also, the other two poles may have the same length.However, when considering the limited inner space of the compressor 10,a permanent magnet having four poles may be limited in arrangement. Thatis, if the four poles are arranged, the permanent magnet may increase inlength, and thus, the linear motor may increase in length.

Thus, the permanent magnet 230 according to embodiments may have twopoles positioned at a central portion to serve as one pole and threepoles that are alternately arranged. Thus, the pole disposed at thecentral portion, that is, the central magnetic pole may have a lengthgreater than a length of each of both end magnetic poles. Thus, whencompared to a case in which four poles are arranged, a compact structuremay be realized. In addition, both end magnetic poles may be reduced bya half or less in length. That is, the following relational expressionmay be defined.

MF or MR≦MC≦2*MF or 2*MR

Also, the central magnetic pole may have a length MC less than a sum ofthe length MF of the first pole 231 and the length MR of the second pole232.

In summary, the greater the length of the central magnetic poleincreases, the more the mutual acting force with the first statormagnetic pole 217 or the second stator magnetic pole 218 increases.Thus, the thrust may increase.

However, when considering a whole size of the linear motor, that is,when considering miniaturization or compactification, if the forgoingrelational expression is satisfied, the two effects, that is, increaseof the thrust and the compactification of the compressor may beachieved.

The length P of the first stator magnetic pole 217 or the second statormagnetic pole 218 in the axial direction may be determined on the basisof stroke S of the piston 130 when a maximum load is applied to thecompressor 10. The stroke S of the piston 130 may be a distance betweenthe TDC position and the BDC position.

When the piston 130 is positioned at the BDC position, a first end (aleft end in FIG. 8) of the first pole 231 may be disposed outside thefirst core 211 a. The first end of the first pole 231 may be defined asan end opposite the first interface surface 235, which defines a secondend of the first pole 231.

Also, outside of the first core 211 a may be understood as an areadefined as outside of a virtual line in a radius direction, which passesthrough an outer end of the first core 211 s. Also, the terms “outside”or “outward direction” may refer to a direction extending away from thecenter of the opening 219, and “inside or inward direction” may refer toa direction toward or closer to the center of the opening 219.

Also, when the piston 130 is positioned at the TDC position, the firstend of the first pole 231 may be disposed inside the first core 211 a.That is, the first end of the first pole 231 may be disposed within aregion, in which the first core 211 a is disposed, with respect to theaxial direction.

However, the first end of the first pole 231 may not move inside of thefirst stator magnetic pole 217. That is, the first end of the first pole231 may be disposed at a position corresponding to an end of the firststator magnetic pole 217 or disposed outside of the first statormagnetic pole 217. Herein, the phrase inside of the first statormagnetic pole 217 may be refer to a space between virtual lines in theradial direction, which pass through both ends of the first statormagnetic pole 217.

The first stator magnetic pole 217 may have the same axial length as thesecond stator magnetic pole 218. In more detail, an axial length P ofthe first or second stator magnetic pole 217 or 218 may be determined byadding a control error or mechanical error to the stroke S of the piston130. For example, if the stroke S is about 16 mm, the length P may beset to about 18 mm.

If the length P is less than the stroke S, the first or second interfacesurface 235 or 236 may move outward from the first or second statormagnetic pole 217 or 218. Thus, the force pushed and pulled between themagnetic poles 217 and 218 and the permanent magnet 230 may be reduced.Thus, the length P may be determined to be greater than the stroke S.

A relational expression between the length P and the length of the firstpole 231 or the second pole 233 is defined. When each of the first andsecond interface surfaces 235 and 236 is reciprocated with respect tothe center of each of the first and second stator magnetic poles 217 and218, if both ends of both end magnetic poles 231 and 233 move into bothends of the first and second stator magnetic poles 217 and 218, thethrust applied to the permanent magnet may be reduced. That is, if atleast a portion of both end magnetic poles 231 and 233 is not disposedoutside both ends of the first and second stator magnetic poles 217 and218, the mutual acting force between the magnetic fluxes of the outerstator 210 and the permanent magnet 230 may be weakened.

Thus, when considering the thrust for generating the reciprocatingmotion of the permanent magnet 230, the length MF of the first pole 231and the length MR of the third pole 233 may be greater than the length Pof each of the first and second stator magnetic poles 217 and 218.

However, the length MF of the first pole 231 and the length MR of thethird pole 233 are factors that have an influence on a whole length ofthe permanent magnet 230. Thus, the lengths MF and MR may be used as alimiting factor to realize miniaturization of the linear compressor 10.

Thus, the current embodiment proposes the following relationalexpression.

MF or MR≧0.9*P

According to the above-described relational expression, if the length MFof the first pole 231 and the length MR of the third pole 233 are withina range similar to the length P of each of the first and second statormagnetic poles 217 and 218, the thrust may be reduced, and the linearcompressor 10 may be compact.

FIG. 10 is a graph showing a magnitude of a thrust generated accordingto lengths of magnetic poles at both ends, in the permanent magnetaccording to an embodiment. FIG. 11 is a graph showing a magnitude of acogging force according to lengths of magnetic poles at both ends, in apermanent magnet according to an embodiment.

Referring to FIG. 10, a change in thrust with respect to a same inputcurrent according to a length of each of both end magnetic poles 231 and233 according to embodiments is illustrated. The horizontal axis in FIG.10 illustrates a position of the permanent magnet 230. A zero point (O)on the horizontal axis may be defined as a state in which each of thefirst and second interface surfaces 235 and 236 is disposed at thecenter of each of the first and second stator magnetic poles 217 and218. This state may be understood as a state in which the permanentmagnet is disposed at the zero point.

Also, a negative (−) position may be defined as a case in which thepermanent magnet 230 moves from the zero point in a first direction, anda positive (+) position may be defined as a case in which the permanentmagnet 230 moves from the zero point in a second direction. Along thehorizontal axis, the more a critical value in position increases, thegreater a distance from the zero point.

Referring to FIG. 10, when the permanent magnet 230 is disposed at thezero point, the thrust may be maximally generated. Also, the more eachof both end magnetic poles 231 and 233 increases in length, the more themaximum thrust may increase.

For example, under a same condition in which the central magnetic pole232 has a length of about 24 mm, if each of both end magnetic poles 231and 233 has a length of about 19 mm, the maximum thrust may be F1 N.Also, if each of both end magnetic poles 231 and 233 has a length ofabout 17 mm, the maximum thrust may be F2 N. Here, the maximum thrustsmay be defined as follow: F1>F2>F3

Also, the more each of both end magnetic poles 231 and 233 increases inlength, the more a magnitude of the thrust may significantly increase onthe whole. That is, as the more each of both end magnetic poles 231 and233 increase in length, the more the magnitude of the thrust applied tothe permanent magnet 230 increases, operation efficiency of thecompressor may be improved.

FIG. 11 illustrates variations or a change in peak value of a force dueto magnetic reluctance of the permanent magnet 230, that is, a coggingforce according to length of each of both end magnetic poles 231 and 233according to an embodiment.

The magnetic reluctance or cogging force of the permanent magnet 230 maybe understood as electrical resistance with respect to an mutual actingforce between the magnetic flux generated in the outer stator 210 andthe magnetic flux of the permanent magnet 230. The cogging force mayincrease to a peak value according to a position (position (+) ornegative (−) position) of the permanent magnet or vary in a direction inwhich the peak value decreases. In more detail, when the permanentmagnet 230 is disposed at the positive (+) position, the cogging forcemay be formed in a positive (+) direction and have a peak value at apredetermined position. On the other hand, when the permanent magnet 230is disposed at the negative (−) position, the cogging force may beformed in a negative (−) direction and have a peak value at apredetermined position. Here, the positive (+) and negative (−)directions of the cogging force may denote forces acting in directionsopposite to each other.

The more the peak value increases, the more the force applied to thesprings 151 and 155 may increase. Thus, it may be difficult to controlthe linear motor 200.

Referring to FIG. 11, the more each of both end magnetic poles 231 and233 increases in length, the more the positive (+) and negative (−) peakvalue of the cogging force may decrease. Thus, the linear motor 200 maybe easily controlled.

For example, under the same condition in which the central magnetic pole232 has a length of about 24 mm, if each of both end magnetic poles 231and 233 has a length of about 19 mm, a peak value of the cogging forcemay be about 15 N. Also, if each of both end magnetic poles 231 and 233has a length of about 18 mm, a peak value of the cogging force may beabout 20 N. Also, if each of both end magnetic poles 231 and 233 has alength of about 17 mm, a peak value of the cogging force may be about 27N.

FIG. 12 is a graph showing a magnitude of a thrust generated accordingto a length of a central magnetic pole, in the permanent magnetaccording to an embodiment. FIG. 13 is a graph showing variations in acogging force according to a length of a central magnetic pole, in thepermanent magnet according to an embodiment.

Referring to FIGS. 12 and 13, the more each of both end magnetic polesincrease in length, the more the thrust may increase, and the peak valueof the cogging force may decrease. As described with reference to FIGS.10 and 11, as the thrust increases, operation efficiency of the linearmotor may be improved. Also, as the peak value of the cogging forcedecreases, the control reliability of the linear motor may be improved.

Referring to FIG. 12, it can be seen that the thrust increases as thecentral magnetic pole increases in length MC under the condition inwhich both end magnetic poles have the same length. For example, underthe condition in which the lengths MF and MR are about 18 mm, it is seenthat the thrust (the maximum thrust: 85 V/m/s) when the length MC isabout 26 mm may be greater than that (the maximum thrust: 83 V/m/s) whenthe length MC is about 24 mm.

Referring to FIG. 13, it is seen that the peak value of the coggingforce decreases as the central magnetic pole increases in length MCunder the condition in which both end magnetic poles may have the samelength. For example, under a condition in which the lengths MF and MRare about 18 mm, it is seen that the peak value (about 13 N) of thecogging force when the length MC is about 26 mm may be less than that(about 20 N) of the cogging force when the length MC is about 24 mm.

According to embodiments, as the permanent magnet may include a magnethaving three polarities, an amount of a magnetic flux generated may beincreased. Also, the increased magnetic flux of the permanent magnet mayinteract with a magnetic flux generated from the outer stator, therebyincreasing a thrust exerted on the piston.

Further, as a length of the opening between the magnetic poles disposedin the outer stator may be maintained equal to or greater than the airgap between the outer stator and the inner stator, it is possible toreduce a leaked magnetic flux and increase a magnitude of the magneticflux generated from the outer stator and oriented toward the innerstator. Accordingly, the air gap magnetic flux and the magnetic flux ofthe permanent magnet may interact with each other, thereby generatinghigher thrust.

Moreover, in the permanent magnet having three poles, a length of theboth-end magnetic pole may be a predetermined proportion of a length ofmagnetic pole of the outer stator, thus making it possible to increase agenerated thrust in comparison with a current applied to the linearmotor and also reduce a cogging force (or torque).

Additionally, in the permanent magnet having three poles, a length ofthe central magnetic pole may be greater than lengths of the both-endmagnetic poles, and may be twice or less than the lengths of theboth-end magnetic poles. This also enables a generated thrust to beincreased and a cogging force (or torque) to be reduced.

Also, the piston and the cylinder may be made of a nonmagnetic material,such as aluminum or aluminum alloy, and thus, magnetic flux may beprevented from being leaked to the outside through the piston orcylinder. In addition, the permanent magnet may be made of aninexpensive ferrite material, thereby reducing a manufacturing cost forthe motor assembly.

Embodiments disclosed herein provide a linear compressor provided with alinear motor capable of generating a sufficient force (a thrust).

Embodiments disclosed herein provide a linear compressor that mayinclude a cylinder that forms a compression space for a refrigerant; apiston that reciprocatably moves in an axial direction inside thecylinder; and a linear motor that supplies a power to the piston. Thelinear motor may include an outer stator including a first statormagnetic pole, a second stator magnetic pole, and an opening definedbetween the first stator magnetic pole and the second stator magneticpole; an inner stator disposed apart from the outer stator; and apermanent magnet movably disposed in an air gap between the outer statorand the inner stator, and having three poles. The three poles mayinclude two both-end magnetic poles, and a central magnetic poledisposed between the two both-end magnetic poles. The central magneticpole may have a length greater than the both-end magnetic poles.

A length of the central magnetic pole may be twice or less than that ofany one of the two both-end magnetic poles. A length of the centralmagnetic pole may be equal to or less than a sum of, lengths of the twoboth-end magnetic poles. An axial direction length of the opening may beequal to or greater than a radial direction height of the air gap.

The piston may be moveable by a stroke between a top dead center (TDC)and a bottom dead center (BDC), and a length of the first statormagnetic pole or the second stator magnetic pole may be equal to or lessthan the stroke. A length of any one of the two both-end magnetic polesmay be approximately 90% or more of a length of the first statormagnetic pole or second stator magnetic pole.

The two both-end magnetic poles may include a first pole coupled to thecentral magnetic pole at a first interface, and a second pole coupled tothe central magnetic pole at a second interface. The first interface mayreciprocate in an axial direction between both ends of the first statormagnetic pole, based on a center of the first stator magnetic pole, andthe second interface may reciprocate in an axial direction between bothends of the second stator magnetic pole, based on a center of the secondstator magnetic pole.

The first pole may include an end at a position facing the firstinterface, and the end of the first pole may be positioned outside theouter stator when the piston is positioned at the BDC. The end of thefirst pole may be positioned at an end of or outside the first statormagnetic pole when the piston is positioned at the TDC.

The two both-end magnetic poles may include a first pole coupled to oneside of the central magnetic pole, and at least a portion of the firstpole may be positioned in an air gap between the first stator magneticpole and the inner stator. The two both-end magnetic poles may include asecond pole coupled to the other side of the central magnetic pole, andat least a portion of the second pole may be positioned in an air gapbetween the second stator magnetic pole and the inner stator.

The opening may be defined between a tip of the first stator magneticpole and a tip of the second stator magnetic pole, at one side of anaccommodation space for accommodating a coil.

The permanent magnet may be made of a ferrite material. The piston andthe cylinder may be made of aluminum or aluminum alloy.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the spirit and scope of the principles ofthis disclosure. More particularly, various variations and modificationsare possible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

Any reference in this specification to “one embodiment,” “anembodiment,” “example embodiment,” etc., means that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention. Theappearances of such phrases in various places in the specification arenot necessarily all referring to the same embodiment. Further, when aparticular feature, structure, or characteristic is described inconnection with any embodiment, it is submitted that it is within thepurview of one skilled in the art to effect such feature, structure, orcharacteristic in connection with other ones of the embodiments.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the spirit and scope of the principles ofthis disclosure. More particularly, various variations and modificationsare possible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. A linear compressor, comprising: a cylinder thatforms a compression space for a refrigerant; a piston that reciprocatesin an axial direction inside of the cylinder; and a linear motor thatsupplies power to the piston, wherein the linear motor comprises: anouter stator comprising a first stator magnetic pole, a second statormagnetic pole, and an opening defined between the first stator magneticpole and the second stator magnetic pole; an inner stator disposed apartfrom the outer stator to form an air gap therebetween; and a permanentmagnet movably disposed in the air gap between the outer stator and theinner stator and having three poles, wherein the three poles include:two end magnetic poles; and a central magnetic pole disposed between thetwo end magnetic poles, wherein the piston is moveable by a strokebetween a top dead center position and a bottom dead center position,and wherein a length of the first stator magnetic pole or the secondstator magnetic pole is greater than a length of the stroke.
 2. Thelinear compressor according to claim 1, wherein the central magneticpole has a length greater than a length of any one of the two endmagnetic poles.
 3. The linear compressor according to claim 2, whereinthe length of the central magnetic pole is twice or less than the lengthof the any one of the two end magnetic poles.
 4. The linear compressoraccording to claim 1, wherein a length of the central magnetic pole isequal to or less than a sum of lengths of the two end magnetic poles. 5.The linear compressor according to claim 1, wherein an axial directionlength of the opening is equal to or greater than a radial directionheight of the air gap.
 6. The linear compressor according to claim 1,wherein a length of any one of the two end magnetic poles isapproximately 90% or more of a length of the first stator magnetic poleor the second stator magnetic pole.
 7. The linear compressor accordingto claim 1, wherein the two end magnetic poles comprise: a first polecoupled to the central magnetic pole at a first interface; and a secondpole coupled to the central magnetic pole at a second interface.
 8. Thelinear compressor according to claim 7, wherein the first interfacereciprocates in an axial direction between both ends of the first statormagnetic pole, based on a center of the first stator magnetic pole, andthe second interface reciprocates in an axial direction between bothends of the second stator magnetic pole, based on a center of the secondstator magnetic pole.
 9. The linear compressor according to claim 7,wherein the first pole comprises an end at a position that faces thefirst interface, and wherein the end of the first pole is positionedoutside of the outer stator when the piston is positioned at the bottomdead center position.
 10. The linear compressor according to claim 9,wherein the end of the first pole is positioned at an end of or outsideof the first stator magnetic pole when the piston is positioned at thetop dead center position.
 11. The linear compressor according to claim1, wherein the two end magnetic poles comprise a first pole coupled to afirst side of the central magnetic pole, and wherein at least a portionof the first pole is positioned in the air gap between the first statormagnetic pole and the inner stator.
 12. The linear compressor accordingto claim 11, wherein the two end magnetic poles comprise a second polecoupled to a second side of the central magnetic pole, and wherein atleast a portion of the second pole is positioned in the air gap betweenthe second stator magnetic pole and the inner stator.
 13. The linearcompressor according to claim 1, wherein the opening is defined betweena tip of the first stator magnetic pole and a tip of the second statormagnetic pole, at a side of an accommodation space that accommodates acoil.
 14. The linear compressor according to claim 1, wherein the pistonand the cylinder are made of aluminum or aluminum alloy.
 15. A linearcompressor, comprising: a cylinder that forms a compression space for arefrigerant; a piston that reciprocates in an axial direction inside ofthe cylinder; and a linear motor that supplies power to the piston,wherein the linear motor comprises: an outer stator comprising a firststator magnetic pole, a second stator magnetic pole, and an openingdefined between the first stator magnetic pole and the second statormagnetic pole; an inner stator disposed apart from the outer stator toform an air gap therebetween; and a permanent magnet movably disposed inthe air gap between the outer stator and the inner stator and havingthree poles, wherein the three poles include: two end magnetic poles;and a central magnetic pole disposed between the two end magnetic poles,wherein the piston is moveable by a stroke between a top dead centerposition and a bottom dead center position, wherein a length of thefirst stator magnetic pole or the second stator magnetic pole is greaterthan a length of the stroke, and wherein the permanent magnet is made ofa ferrite material.
 16. The linear compressor according to claim 15,wherein the central magnetic pole has a length greater than a length ofany one of the two end magnetic poles.
 17. The linear compressoraccording to claim 15, wherein a length of the central magnetic pole isequal to or less than a sum of lengths of the two end magnetic poles.18. The linear compressor according to claim 15, wherein an axialdirection length of the opening is equal to or greater than a radialdirection height of the air gap.
 19. The linear compressor according toclaim 15, wherein a length of any one of the two end magnetic poles isapproximately 90% or more of a length of the first stator magnetic poleor the second stator magnetic pole.
 20. The linear compressor accordingto claim 15, wherein the two end magnetic poles comprise: a first polecoupled to the central magnetic pole at a first interface; and a secondpole coupled to the central magnetic pole at a second interface.
 21. Thelinear compressor according to claim 20, wherein the first interfacereciprocates in an axial direction between both ends of the first statormagnetic pole, based on a center of the first stator magnetic pole, andthe second interface reciprocates in an axial direction between bothends of the second stator magnetic pole, based on a center of the secondstator magnetic pole.
 22. The linear compressor according to claim 20,wherein the first pole comprises an end at a position that faces thefirst interface, and wherein the end of the first pole is positionedoutside of the outer stator when the piston is positioned at the bottomdead center position.
 23. The linear compressor according to claim 22,wherein the end of the first pole is positioned at an end of or outsideof the first stator magnetic pole when the piston is positioned at thetop dead center position.
 24. The linear compressor according to claim15, wherein the two end magnetic poles comprise a first pole coupled toa first side of the central magnetic pole, wherein at least a portion ofthe first pole is positioned in the air gap between the first statormagnetic pole and the inner stator, and wherein the two end magneticpoles comprise a second pole coupled to a second side of the centralmagnetic pole, and wherein at least a portion of the second pole ispositioned in the air gap between the second stator magnetic pole andthe inner stator.
 25. The linear compressor according to claim 24,wherein the piston and the cylinder are made of aluminum or aluminumalloy.